In June 2005, the World Business Council for Sustainable Development defined 5 Key Performance Indicators within the Cement Sustainability Initiative [J. S. Damtoft et al.]. These indicators cover various subjects like energy reduction and an increasing use of alternative raw materials, as well as the reduction of CO2-emissions that is becoming a key issue in the sustainable development of most companies [H. Mikulcic et al.]. The high temperatures needed to obtain optimal reactivity in clinker kilns, typically about 1500° C., are highly energy-consuming. The raw clinker meal is in particular to be molten partially to provide a liquid phase wherein sintering processes take place. In the current processes, the required energy is delivered by burning both traditional and alternative hydrocarbon fuels as well as and recovered fuel materials. Despite the significant CO2-emissions related to the combustion of these fuel materials, CO2-emissions are mainly generated by the use of limestone as the main raw material for clinker. Limestone, primarily consisting of CaCO3, will release CO2 during decarbonation, delivering CaO as main constituent of alite (3CaO.SiO2), belite (2CaO.SiO2), aluminate (3CaO.Al2O3) and ferrite (4CaO.Al2O3.Fe2O3) which are the major clinker phases.
The use of alternative fuels based on recycled energy-rich materials or energy-rich non-marketable by-products of e.g. chemical processes is already well established in the cement industry. For example, U.S. Pat. No. 5,614,016 (F. L. Smidth & Co A/S, Denmark, 1997) and US 2007/0122762 A1 (Buzzi Unicem, USA, 2007) disclose the use of several kinds of waste materials which are substantially completely combustible and exothermic, preferably tires and plastics, mixed with the raw material feed and fed into a pre-calcining stage of a cement clinker production process.
On the other hand, the use of alternative raw materials based on recovered materials as replacement for limestone is less frequently implemented. Nevertheless, the replacement of limestone could minimize the effects of quarrying and improve the environmental impact by energy reduction as well as the reduction of CO2 emission, by acting as a non-carbonate CaO source.
US2005/0066860 A1 (T. J. Logan et al. USA, 2005) discloses the use of a mixture of organic waste material (preferably sewage sludge) and mineral by-products (preferably a coal combustion by-product such as fly ash, bottom ash, fluidized bed ash, boiler ash and flue gas desulfurization by-products) in a cement manufacturing process, both as a fuel and combined with the raw material feed to the kiln. The inorganic material content is about 25 to 75 weight %. The mixture is used as a raw material feed in an amount of 10 to 50 weight % based on the total weight of the raw material feed. A fibre cement material is not mentioned, nor a method of production of a cement clinker.
The use of fibre cement material, wherein the fibres don't comprise organic fibres but asbestos fibres, in the production of cement clinker is disclosed in the article “Cement-bound asbestos products successfully disposed of and recycled in the clinker burning process”, ZKG International, Bauverlag B V., Getersloh, D E, vol. 48, no. 11, 1 Jan. 1995. Shredded asbestos cement sheets were fine-ground to particle sizes corresponding to a cement raw meal. The asbestos cement meal was injected in the flame of a rotary kiln through the inner pipe of a multi-channel burner in a quantity corresponding to an input of 2% of the raw meal feed to a kiln plant. Due to the very high temperature of more than 1600° C., the asbestos components were destructed. Moreover, no asbestos fibres could be detected in the exhaust gas dust. A drawback of such a destruction method is that the heat exchanging properties of the cyclone tower are not used to preheat the asbestos cement material resulting in higher energy requirements of the kiln, especially when larger amounts of asbestos fibres would be used. Indeed, at the flame of the rotary kiln a certain maximum temperature should be achieved in the clinker production process whilst the addition of relatively cold asbestos cement in this zone of the kiln reduces the temperature in that maximum temperature zone.
In order to avoid fine milling of the fibre cement material, in particular asbestos cement material, WO 98/02392 proposes to only crush the fibre cement material in order to obtain a coarser granulate which can be fed via the inlet feed end into the rotary kiln. Due to the high temperature of about 800° C. at this inlet, no non-destroyed asbestos fibres were exhausted from the kiln.
A method, wherein organic fibre cement material is used as a raw material in the production of cement clinker in a cement manufacturing process as a supplemental source of raw material up to 2 weight % of the total weight of the meal, has been disclosed in Global Cement Magazine, October 2010, pp. 12-16. It is described that the fibre cement material is fed into a hopper from where it is fed into the feeder of a raw clinker meal mill, i.e. it is mixed with and processed along with the conventional clinker meal. At said concentration of 2 weight %, it was concluded that the quality of the clinker still met all quality requirements, especially in terms of amounts of Al, Si, Ca and Fe, and to a lesser extent Mg, K, Na and S, and the emissions to the atmosphere at the kiln's chimney still met all emission limiting values as required by the exploitation permit. An advantage of this method is that by adding to organic fibre cement waste material to the raw clinker materials before grinding them to produce the raw clinker meal, the organic fibre cement material is also preheated in the cyclone tower (which is heated by the kiln gas evacuated from the rotary kiln to recuperate the heat thereof) so that the fibre cement waste material is added, in the same energy efficient way as the other raw materials, to the inlet at the top of the heat exchanging cyclone tower.
The current application discloses however the effects of using large amounts of recuperated organic fibre cement materials as alternative raw material for Portland clinker production and further describes possible boundary conditions and/or limitations. In particular adding larger amounts of organic fibre cement material either to the raw clinker meal feed or directly into the rotary kiln appeared to produce organic compounds which increase the total organic carbon (TOC) content of the exhaust gasses from the clinker production installation and/or which may be deposited onto the inner walls of the cyclone tower (pre-heater).